Method and apparatus for separating fluid components

ABSTRACT

A floating element for separating components of a physiological fluid comprises two parts that are relatively movable. The two parts define a prescribed volume between them when at their maximum separation, and one of the parts may be moved toward the other to express the fluid contained in the volume between the parts. The parts are made of materials having densities so that they assume a desired position in the fluid to allow selected components to be easily obtained and expressed.

TECHNICAL FIELD

This invention relates to a unique element that floats in aphysiological fluid undergoing centrifugation and assumes a locationencompassing the boundary region between two components of differentdensities, and facilitates the isolation of a desired component found inthe boundary region. Specifically, the floating element greatlyfacilitates the isolation and separation of the buffy coat from plasmaand red blood cells.

Another aspect of the invention is the provision of a device for useboth as a syringe for withdrawing physiological fluids and as a chamberfor separating the components of the fluids. In its preferredembodiments, the invention is a syringe configured to withdraw fluidsfrom a patient in known fashion and subsequently to be placed directlyin the rotor of a centrifuge for separating components of differentdensities. The syringe is thereafter operated to express the componentsin serial fashion, for example, into separate cups.

BACKGROUND ART

Processing physiological fluids by centrifugation for separating thefluids into components of different densities is known. Physiologicalfluids include, for example, peripheral blood, umbilical cord blood, andbone marrow aspirate and ordinarily include cellular components. Thephysiological fluids subjected to the processes described herein may beobtained directly from a patient being treated, in which case the fluidsare autologous, obtained from a donor, or obtained from a plurality ofdonors, in which case the fluids are homologous. While the objects ofthe invention are primarily concerned with the treatment of humanfluids, it will be appreciated that the methods and apparatus describedherein are equally applicable to fluids from other species.

A primary objective of the invention is to isolate and obtain a layer ofcells that forms during centrifugation and includes, among othercomponents, platelets, white cells, stem cells, and nucleated cells.This layer is known as the buffy coat, and its density is between thatof the red blood cells (1.08-1.09) and that of plasma (1.017-1.026).Plasma with most of the cellular components removed is known as plateletpoor plasma (PPP), while plasma with its cellular components is known asplatelet rich plasma (PRP). Platelet rich plasma has been found toproduce several beneficial effects, such as a more rapid healing ofwounds. Thus, another objective of the invention is to provide plasmawith an increased level of platelets. This is known as a plateletconcentrate (PC) or more broadly as a cell concentrate (CC). A typicalconcentration is four or more times the native concentration, and atypical ratio of input volume to cell concentrate volume is 6:1.Platelet or cell concentrates obtained by the invention comprises thebuffy coat and plasma and may include a small amount of red blood cells.

One of the problems addressed by the present invention is that thespecific proportion of the various components and, even, the density ofthe cells themselves are unique to the particular donor, which precludesan exact a priori determination of the location of any given componentin the fluid after centrifugation. For example, the proportion of redblood cells in blood, the hematocrit, varies with each patient, and theaverage density of the red blood cell component varies with itsproportion of neocytes, young red blood cells, whose density is lessthan 1.08.

Furthermore, the particular technique used to collect the fluids impactsthe density of the cells. An anticoagulant is typically added to bloodas it is collected, and the amount of anticoagulant and the particularanticoagulant used affects the density, particularly, of red bloodcells. This is termed the lesion of collection and results from theeffect of the anticoagulant on the osmolarity of the cells. For example,when the anticoagulant is acid citrate dextrose, ACD, red cells becomehypo-osmolar and the cells draw water through the dell membrane, whichdecreases the density of the cells. Other anticoagulants, such astri-sodium citrate at a concentration of 3.8%, are somewhathyper-osmolar, which results in shrinkage of the red blood cells and anincrease in their density. CPD is iso-osmolar and has much less effecton the density of the cells. CPD and tri-sodium phosphate are preferredand have produced superior results in separations of the kindcontemplated herein.

A further factor is that the layers of components form along the radiusof centrifugation and are thus cylindrical, which complicates the designof structural elements for separating or collecting the layers.

A system for separating blood into components for producing a plateletconcentrate is described in U.S. Pat. No. 6,398,972. The systemdescribed in that patent uses a disposable processing unit having twochambers. Blood is drawn into a known syringe and expressed from thesyringe into a first chamber of the processing unit. The processing unitis then placed in a centrifuge designed to automatically transfersupernatant fluids from one chamber to another. After a firstcentrifugation, platelet rich plasma is transferred into the secondchamber, and the centrifuge is operated a second time to separateplatelets from platelet poor plasma. While this system has manyadvantages, it has the disadvantage that the blood must be transferredfrom the syringe to the processing unit, and the centrifuge and theorientation of the processing unit must be controlled to decant theplatelet rich plasma to the second chamber.

The first chamber of the system described in the '972 patent includes adisk that is positioned generally at the intersection of the red bloodcells and the plasma to prevent decanting of red blood cells into thesecond chamber.

U.S. Pat. No. 5,456,885 shows a system wherein a collection tube isplaced directly in a centrifuge to allow separation of the components. Afloating element assumes a position between the plasma and the red bloodcells and also acts as a check valve when the lighter phase is expressedfrom the tube. Systems of this type are, however, not generally capableof separating the buffy coat from the platelet-poor plasma and the redblood cells.

The known centrifuges operate according to a particular protocol when itis desired to obtain a component of intermediate density. For example,when the object is to obtain platelets, it is known to subject blood toa first centrifugation to separate heavier components, such as red bloodcells, from plasma, transferring the plasma to a second container orchamber by decanting and then subjecting the plasma to a secondcentrifugation to separate the plasma from the platelets. The plateletsare then separated from the plasma in a second decanting step.

Known techniques for obtaining the desired component of intermediatedensity are complicated because they require multiple centrifugationsand multiple decant or centrifugal transfer steps. Also, the separationof a single component is often complicated because the physical, fluidproperties of the desired component may tend to cause it to mix with theother components.

The buffy coat layer is easily disrupted, and when one attempts toexpress platelet poor plasma through the tip of a syringe, the buffycoat often mixes with the plasma or with the red blood cells. Thiseffectively prevents the expressing of the buffy coat layer either byitself or with only a negligible amount of the other components.

As well, known tubes or syringes designed to be supplied directly to acentrifuge are difficult to use effectively.

SUMMARY OF THE INVENTION

In accordance with the invention, an improved device is provided forseparating components having differing densities in a centrifuge andisolating and dispensing a desired one of these components. The devicemay take the form of a syringe in the sense that it can be provided witha plunger and operated to draw a fluid, such as blood, bone marrowaspirate, or other physiological fluids, into a chamber through one endand to express the components through that end after separation. Thedevice may, however, be a container of other configurations capable ofbeing placed in a centrifuge and not necessarily designed to operate asa syringe.

A particular objective of the invention is to obtain a cell concentratefrom whole blood (including umbilical cord blood), bone marrow aspirate,or other physiological fluid in an efficient fashion throughcentrifugation and the expressing of the several components. The cellconcentrate preferably includes the buffy coat, some red cells, andplasma in desired ratio. The buffy coat is a thin layer that formsduring centrifugation and includes mostly all of the cells other thanthe red blood cells. The buffy coat is known to include platelets, whitecells, nucleated cells, and stem sells cells and may include othercomponents as well. Because the buffy coat is a somewhat diffuse layerthat is easily disrupted and mixed with the other components, whichreduces the effectiveness of the procedure, an object of the inventionis to provide a container that can be operated to dispense the cellconcentrate without significant mixing of the desired cells with theplasma or the red blood cells. This is accomplished in the preferredembodiments primarily by providing a flow path for the cell concentratethat reduces mixing between the components. In the preferredembodiments, a disk assembly floats in a region containing an interfacebetween plasma and the buffy coat and a diffuse interface between thebuffy coat and the red cells, and assists in separating thosecomponents. As well, the disk assembly is shaped so that it forms a flowpath for the components and reduces turbulence during separation of thecomponents to prevent mixing the components during their expression.

In its preferred embodiment, a disk assembly that is allowed to float inthe fluid presents a vertical gradient in the buoyant forces that causeit to assume a position in the region having the desired component,e.g., the buffy coat. This gradient is provided either by the shape ofthe assembly, by the use of materials of different densities, or by acombination of both. In the preferred embodiment, the disk assemblyprovides a conical upper surface, and an upper portion of the assemblyis made of a material that is less dense than red blood cells but moredense that plasma. A lower portion of the assembly is made of a materialthat is denser than the red blood cells. Because of the conical shape,the buoyant force provided by the upper element at the boundary betweenthe plasma and the red blood cells and in the region of the buffy coatis a non-linear function of the distance by which the upper elementextends into the plasma. The density gradient of the fluids in theboundary region is large, and the use of a floating element with adensity gradient also has been found to be beneficial.

The disk assembly according to the invention is designed to encompassboth a desired component and a predetermined volume of fluid surroundingthe desired component. In the preferred embodiment, the disk assemblycomprises two floating parts that are movable relative to each otherwhereby the entire assembly is caused to assume a desired position aftercentrifugation, and one part moves toward the other during expression ofthe fluids to express a desired component or components, e.g., the buffycoat and a predetermined volume of plasma. This structure allows theuser to obtain a cell concentrate comprising the buffy coat mixed withplasma at a desired increased concentration.

The invention also relates to perfecting mechanical features, such as ahandle for a plunger that accommodates placing the syringe in acentrifuge, and a stand for holding the syringe after centrifugation forfacilitating expression of the components. The handle may be detachableor flexible whereby the distance by which it extends from the end of thebarrel when the syringe is full is greatly reduced.

It is an object of this invention to provide a device for use inseparating the various components of a fluid by placing the fluid in thedevice, subjecting the device and fluid to centrifugation, and thenexpressing the components.

It is a further object of this invention to provide a syringe forwithdrawing fluids from a container or from the patient, for beingplaced directly into a centrifuge, and for expressing the separatedcomponents in serial fashion with minimal mixing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross section of a first embodiment of a syringeaccording to the invention.

FIG. 2 is a vertical cross section of a second embodiment of a syringeaccording to the invention.

FIGS. 3 a and 3 b are vertical cross sections of a third embodiment of asyringe according to the invention.

FIGS. 4 a and 4 b are vertical cross sections of a fourth embodiment ofa syringe according to the invention.

FIG. 5 is a side view of a stand, or holder for engaging a syringe ofthe invention.

FIGS. 6 a and 6 b are side views of a syringe according to the inventionshowing a first embodiment of plunger handles that can be detached.

FIGS. 7 a and 7 b are side views of a syringe according to the inventionshowing a second embodiment of plunger handles that can be detached.

FIGS. 8 a and 8 b illustrate a further embodiment of a detachableplunger.

FIG. 9 shows yet another embodiment of a detachable handle.

FIG. 10 a illustrates a syringe having a ring that retains a piston, and

FIG. 10 b illustrates a plunger handle that has been modified toaccommodate the ring during assembly.

FIGS. 11 a and 11 b show vertical cross sections of a preferred seal forthe plunger of a syringe.

FIG. 12 illustrates a syringe according to the invention having a capassembly that is attached during centrifugation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described below in accordance with its operationas a syringe. Use with a syringe is advantageous because it allows theuser to withdraw the physiological fluid into the syringe, place thesyringe directly into a centrifuge for centrifugal processing, and thento express the several components from the syringe into separatecontainers. As such, this procedure requires only a single containerwithout intermediate decanting steps. It will be understood, however,that many features of the invention do not require operation with asyringe or a single container and that the disk assembly to be describedbelow may be used in combination with other containers as well.

A floating element that automatically assumes a position just below thebuffy coat is disclosed in WO 01/83068. The disk disclosed there isuseful to separate the components of physiological fluids bycentrifugation and finds its primary utility in structures that separatethe components after centrifugation by decanting. The disk shown thereis useful, however, to separate components when used with a structuresuch as a syringe, and several examples of such use are described inU.S. Provisional Patent Application 60/471,352, the disclosure of whichis hereby incorporated by reference.

A primary objective of the present invention is to facilitate theproduction of a cell concentrate having a defined volume of plasma andat least a major portion of the buffy coat. With reference to thedrawings, FIG. 1 shows a preferred embodiment of a disk assembly incombination with a known syringe, which includes a cylindrical barrelportion 2, a conical end portion 4, and a tip portion 6. The syringealso includes a piston or plunger 8, which fits tightly in thecylindrical barrel and moves longitudinally to draw fluids into thebarrel or express fluids from the barrel. A handle 10 is attached to theplunger and is typically configured so that an operator can easily graspit to move the plunger within the barrel. This structure is commonlyfound on syringes that can be obtained from many sources.

In accordance with the embodiment shown in FIG. 1, a disk assemblycomprises an upper element 120 and a lower element 122 that is slidinglymounted on a pin 124 for relative movement with respect to element 120.A third element 126 primarily provides lateral stability to the diskassembly and is mounted on the pin 124 in a fixed position. Thus, in theembodiment of FIG. 1, the three elements float in the fluid and assume apredetermined position s will be described below. It will be appreciatedthat since element 122 is movable along the pin that some lateralstability can be lost when the distance between the upper element 120and the movable element 122 is small and that the presence of the washerelement 126 maintains stability. Other means for providing suchstability such as longitudinally extending skirts may be used as well.

It will also be appreciated that the presence of the washer 126determines the maximum distance by which the element 122 can bedisplaced downward from the upper element. That maximum distance may bedetermined by other means, such as by a stop on the pin 124.

In the preferred embodiment, the disk assembly comprising elements 120,122, and 126 is made of materials selected and configured such that thewasher 126 assumes a position near the top of the layer of red bloodcells. Preferably, the assembly is designed so that the “buffy coat,”lies just on the upper surface of the movable element 122, possibly witha few red blood cells also. This arrangement works particularly wellwhen the objective is to separate platelets, stem cells, white cells,and other cells from the red blood cells and plasma. The primary use ofthe preferred embodiments of the invention is to separate, red bloodcells, plasma, and the mix of cells typically found in the buffy coatfrom whole blood, bone marrow, bone marrow aspirate, or umbilical cordblood, and the embodiments of this invention will be described below inthat context. It should be noted, however, that the methods and devicesof the invention could be used to separate other fluids into componentshaving different densities.

The operator first draws blood into the syringe by pulling the pistonaway from the conical end. The tip of the syringe may be connected toany one of several types of sources, and in the preferred embodiment,the syringe is attached to a needle so that blood is drawn into thesyringe directly from a patient. The blood may be drawn from otherplaces, such as a bag of blood obtained from the patient. The syringehaving blood therein is then placed in a centrifuge and subjected tocentrifugation to cause the components of differing densities toseparate into layers along the barrel. While the preferred structure bywhich the syringe is placed in the centrifuge will be described indetail below, it will be appreciated that a variety of structures may beused to attach the syringe to the rotor of a centrifuge.

In accordance with a preferred embodiment, during centrifugation thedisk assembly eventually assumes a position between red blood cells andplatelet poor plasma, where the buffy coat 14 lies on the upper surface123 of the element 122. When the centrifugation is stopped, plateletpoor plasma 12 will be the upper layer (up being the orientation withthe tip of the syringe pointing upward), red blood cells 13 will be thebottom layer, and the buffy coat 14 will be the intermediate layer. Itwill be understood that the layers as illustrated in FIG. 1 are notexact, particularly because the boundary between red blood cells and thebuffy coat is diffuse.

It will also be appreciated that the distance between the upper element120 and the movable element 122 determines volume of plasma 12 capturedbetween these elements. Thus, after centrifugation, plasma will surroundthe upper element 120, and red blood cells will surround the washer 126and extend onto a small part of the upper surface of the movable element122. After centrifugation, the user pushes on the handle 10 to expressesthe components in serial fashion.

The buffy coat is quite thin, and a common problem faced when expressingthe components is that turbulence occurring during expression, whichcauses the buffy coat to mix with the plasma and red blood cell layers.Thus significantly reduces the ability to express the buffy coat as aseparate component or layer. In accordance with a primary aspect of theinvention, the internal structure of the syringe is designed to providea pathway for the fluids being expressed that avoids mixing thecomponents. Because the buffy coat lies on the upper surface of the diskin the preferred embodiments, the configuration of the disk ispreferably designed to cooperate with the internal surfaces of thesyringe to provide the desired pathway.

The top surface 121 of the upper element 120 is preferably conical toconform to the shape of the conical end 4 of a syringe, whereby it willengage the end of the syringe during expression of the fluids. Othershapes are, of course possible. The upper element 120 includes aflexible seal 128, which is preferably a thin annulus made of plasticthat is flexible enough to allow cells to pass it during centrifugationbut to resist that in normal handling of the syringe. In the preferredembodiment shown in FIG. 1, the upper element is made of two parts, andthe seal rides freely in groove 130 formed between the two parts. Themovable element 122 is also made of two parts and includes a seal 132that rides in groove 134.

The flexible seals 128 and 132 reduce mixing of the components duringexpression and handling, when the syringe may be placed in differentorientations, e.g., when the user lays it horizontally on a table. Thus,the seals prevent flow of the red blood cells from below the seal 132into the predefined area between the upper and movable elements, whichwould reduce the effectiveness of the separation of the components. Theprovision of the seals also increases the allowable manufacturingtolerances and greatly reduces the possibility that deformations in thesyringe barrel during operation of the syringe will adversely affect theoperation of the device.

Element 122 includes several features that allow it to assist inpositioning the disk assembly itself such that a small layer of redblood cells 13 lies just above the upper surface 123 of the element 122and below the buffy coat 14. This ensures that the entire buffy coat isobtained and facilitates the expression of the buffy coat because thered blood cells tend to prevent attraction between the buffy coat andthe upper surface of the movable element. Also, because the boundarybetween the buffy coat and the red blood cells is diffuse, it is notgenerally possible to obtain the entire buffy coat without including asmall amount of the red blood cells.

The movable element 122 preferably includes a vertical density gradientprovided by an upper part 136 having a density of about 1.04 and a lowerpart 138 having a density of approximately 1.08. By this construction,the two parts of the movable part both tend to sink in the plasma, theupper part, however, floats in the red blood cells, and the lower partsinks in red blood cells. In addition to the density gradient presentedby the use of the two materials, it will be appreciated that the conicalshape of the element 136 causes the gradient of the buoyant forces to benon-linear at the boundary between the plasma and red blood cells. Thishas been found to increase the ability of the floating element toposition itself such that the buffy coat lies either on the conicalsurface 123 or just above it.

In operation, fluids are drawn into the syringe, and some air is alsodrawn in. After centrifugation, the air will form a bubble at the top ofthe syringe. If the syringe is then inverted to express the plateletpoor plasma between the element 120 and the end of the syringe, thebubble will move to a position between element 120 and the plasma. Atthat point, the user may express the platelet poor plasma into a cup.The user will know that the platelet poor plasma has been expressed whenthe air bubble 15 reaches the tip of the syringe. At that point the usercan express the air until the upper end of the element 120 contacts theconical end of the syringe. At that point, further movement of theplunger will cause the red blood cells below the element 122 to moveelement 122 upward to express the plasma and buffy coat that lie betweenthe elements 120 and 122. These materials will flow through a channel146 between the pin 124 and the lower part of element 120 and thenthrough a hole 148 in the upper part. As the plunger is advanced, theelement 122 will move upward until all of the material between elements120 and 122, namely the plasma and the buffy coat, and a few red bloodcells in the preferred embodiment, has been expressed. At this point,the force required to advance the plunger will increase significantlybecause the red blood cells will have to move past the seals 132 and128. The user will notice this increase in force and will recognize itas indicating that all of the material has been expressed.

While the syringe is usually cylindrical, it may have other shapes. Forexample and oval cross section may be useful to prevent rotation of theparts.

FIG. 2 illustrates an embodiment where the disk assembly comprises upperpart 68 and lower part 70. The upper and lower disks form a cavity 72between them, and the densities of the upper and lower parts are chosensuch that the buffy coat 14 lies in the space 72. The upper and lowerparts may be attached together by circumferentially spaced strips 74that allow fluids passing between the periphery of the lower part 70 andthe wall 2 of the barrel portion to flow into the cavity 72.

The embodiment of FIG. 2 is advantageous because it captures the buffycoat in the relatively small space 72 between the upper and lower parts68 and 70. This space is preferably made to be sufficiently small thatthe buffy coat will not mix with the plasma even if the syringe is notmaintained in a strictly upright orientation or if the syringe isotherwise tilted or moved about in a fashion that would cause mixing inthe embodiments described earlier. It will be appreciated that it isgenerally necessary to maintain the syringe upright so that thecomponents that are to be dispensed separately are not mixed with eachother. Thus, the embodiment of FIG. 2 is aimed at reducing therequirement for handling the syringe with extreme care not to mix thecomponents.

The buffy coat is expressed in the embodiment of FIG. 2 much the same asin the other embodiments. As the piston or plunger of the syringe isadvanced, the plasma is expressed first until the skirt portion 44engages the syringe, and the red blood cells then flow into the cavity72 as shown at 76 and flush the buffy coat upward through opening 20 andthrough the tip 6. It will be appreciated that the upper part 68 of thedisk shown in FIG. 2 includes a skirt 44 that engages in the annularportion 48. Also, the upper end of the syringe is flat, and the uppersurface of the upper element 68 is also generally flat to reduce thevolume between these two when the disk is in the uppermost position. Itwill be understood, however, that the upper surface of the disk 68 couldbe conical if the upper end of the syringe were conical.

Also, the upper surface of the disk can contact and seal against theinner surface of the syringe.

FIGS. 3 a and 3 b illustrate another embodiment that has a two-partdisk. In this embodiment, the upper part 78 of the disk may move withrespect to the lower part 80. Thus, the lower part 80 is provided with askirt 44 and openings 82 to allow fluid to flow radially inward duringexpression of the buffy coat. The upper part 78 is made of material lessdense than plasma (e.g., LDPE) and is supported on the skirt by elements84 that allow vertical movement of the upper element along the skirt. Ofcourse other arrangements may be provided to provide this motion. Theresult is that the buffy coat will be contained in the cavity 72 betweenthe two elements. Because the upper element floats in the plasma, thespace 72 will initially be larger than required to contain only thebuffy coat. Both parts of the disk will move upward during expression ofthe plasma, and the upper part will eventually engage the end of thesyringe. As the plunger is moved further upward, the lower part willmove upward further as illustrated in FIG. 3 b while the upper part isconstrained against further movement. This will reduce the size of thespace 72 and express the plasma and then the buffy coat from thesyringe.

FIGS. 4 a and 4 b show yet another modification wherein an upper part 86and a lower part 88 of the disk are configured to provide an annulartrough for receiving the buffy coat. These two parts may be configuredto move with respect to each other as in the embodiment of FIG. 3. Inthe embodiment of FIG. 4 a the upper part 86 has a protruding annularportion 90, and the lower part has an annular trough 92 that matches theportion 90. The trough 92 receives the buffy coat, and when the fluidsflow radially inward during expression the buffy coat is expressed asillustrated by the arrows.

FIG. 4 b shows a similar concept where the surfaces 90 and 92 areserpentine.

The previous discussion has not assumed any particular mechanism foradvancing the piston or plunger. FIG. 5 shows an optional stand designedto hold a syringe and facilitate expression of the components. The standincludes a base 94 that includes a vertical rod 96 on which the syringeis placed so that the rod contacts the piston or plunger. The syringe isengaged by a movable carriage 98, which is fitted to the base bycoacting elements to ride vertically on the base. The user's fingers maygrasp a handle 100, and the user's thumb can engage the top of thecarriage 98. Thus, the user can push the syringe downward against therod to move the piston upward and express the components. A tube 102 isconnected to the tip of the syringe to direct the components to thedesired container, such as small cups for receiving, for example,platelet poor plasma and the buffy coat. An optical element may bemounted on a projection 104 to provide audible or other signalsregarding the boundaries of the components to be expressed.

FIGS. 6 and 7 illustrate alternate designs for the syringe whereby thehandles are easily removed. For example, the handles should be removedbefore the syringe is placed in a centrifuge. After centrifugation, thesyringe may be placed in a stand such as that shown in FIG. 5, thehandle reinstalled, or the like. As shown in FIG. 6 a, the plunger 8 mayinclude a tab 106 that engages a hook 108 or similar element on the endof the handle 10. The hook may be disengaged when the piston is fullywithdrawn as shown in FIG. 6 b. FIGS. 7 a and 7 b show a similararrangement except that the lower surface 110 of the hook is angledwhereby it automatically disengages. Thus, when the handle is not fullywithdrawn, the side of the handle engages the barrel portion of thesyringe and prevents disengagement. When the handle is fully withdrawn,the handle is then able to move transversely with respect to the barrel,and application of a longitudinal force to the handle, as is normal whenwithdrawing the piston, automatically applies a transverse force to thehook, causing it to disengage as shown in FIG. 6 b.

FIGS. 8 a and 8 b show additional details of the structure of the hook108. The hook shown in these figures is reinforced by a flange 118,which stiffens it against bending.

FIG. 9 illustrates a hook that has a reinforcing rib 119 that isreceived in a slot 121 in the plunger

Alternatively, the handle may be configured such that its shape can bechanged such that the syringe may be placed in the centrifuge. As oneexample, the handle may be configured so that it can be bent to aposition that allows the syringe to be placed in the centrifuge.

FIG. 10 a illustrates structure that facilitates construction of asyringe according to the invention. To assemble the syringe, the diskassembly and the plunger 8 must be inserted into the barrel 2. A stopmust be provided to prevent withdrawal of the plunger when the syringehas been filled with blood. Thus, as shown in FIG. 10 a, a ring 112 isprovided that is attached to the end of the barrel. Preferably, the ring112 is spin-welded to the barrel during manufacture. FIG. 10 b shows ahandle 10 with a depression 114 that provides space for the spin-weldingmachinery to be attached to the ring 112 during manufacture.

FIGS. 11 a and 11 b are vertical cross-sections of a preferred form ofseal for the plunger seal 140. The seal is made of resilient materialand engages the upper wall of the plunger. A vertical section extendsover the edge of the upper wall and downwardly toward a lower wall. Whenthe syringe is not undergoing centrifugation, the upper surface of theseal is generally concave upwardly, and edge portions 142 engage theinterior surface of the syringe barrel 2. Also, the bottom of the sealis displaced from the lower wall by a gap 144. This provides an adequateseal to prevent leakage when the contents of the syringe are subjectedto pressures near atmospheric. When the syringe is subjected tocentrifugation, however, the pressure applied by the fluid on the seal140 is greatly increased, and additional sealing capability is required.Thus, the seal 140 is designed to deform as shown in FIG. 11 b byapplication of increased pressure by the fluids whereby the lower edgeis made less concave, which presses the edge 142 outward against thewall 2 of the syringe with increased force increasing the sealingcapability. The increase in the diameter of the seal due to itsflattening out is accommodated by a reduction in the size of the gap144.

FIG. 12 illustrates a further improvement. During centrifugation, thecentrifugal forces on the fluids, the plunger, and the syringe barrelare strong enough that the plunger will naturally move outward slightly.That will cause some air to enter the barrel cavity. When thecentrifugation terminates, the elements may recover their initialpositions, which causes expression of the air. To allow this to occurwithout compromising sterility, a cap 116 of hydrophobic material isplaced over the end of the tip 6 after blood has been drawn into thesyringe. This provides a barrier to entry of bacteria after the bloodhas been drawn into the syringe and prevents discharge of blood from thesyringe during handling and centrifugation.

Modifications within the scope of the appended claims will be apparentto those of skill in the art.

1. A disk assembly for use in centrifugation comprising a first andsecond elements arranged for relative movement and made of materialshaving respective densities such that said first element floats abovesaid second element in a fluid, and said second element assumes apredetermined position in said fluid.
 2. A disk assembly according toclaim 1 further including a pin extending from said first element andcarrying said second element.
 3. A disk assembly according to claim 2further comprising a third element below said second element forproviding stability to said assembly. 4-8. (canceled)